Field of the Invention
The present invention relates to an oscillatory wave drive device and an optical apparatus having the same.
Description of the Related Art
An oscillatory type (oscillatory wave) actuator has a vibrator in which drive oscillation is excited in an elastic body of an annular shape, a long oval shape, a rod shape, or the like by applying electric signals, such as an alternating voltage, to an electromechanical energy conversion element, such as a piezoelectric element. As a specific oscillatory type actuator, it has been suggested to utilize the same as an oscillatory wave motor which relatively move the elastic body which is brought into contact with the vibrator under pressure and the vibrator, for example.
The outline of an annular-shaped oscillatory wave motor is described below as an example.
The annular-shaped oscillatory wave motor has an annular-shaped piezoelectric material having a bore diameter and an outer diameter in which the entire circumferential length is an integral multiple of a certain length λ. A plurality of electrodes are provided on one surface of the piezoelectric material and a common electrode which are common to the piezoelectric material is provided on the opposite surface, whereby a piezoelectric element is formed.
The plurality of electrodes contain two drive phase electrodes, a detection phase electrode, and non-drive phase electrodes. Electric fields are applied to the piezoelectric material of each drive phase electrode unit at a λ/2 pitch alternately in opposite directions, so that polarization treatment is performed. Therefore, the polarity of expansion and contraction of the piezoelectric material with respect to the electric fields in the same direction are opposite to each other at each λ/2 pitch. The two drive phase electrodes are disposed while providing interval portions of odd number times of λ/4. In usual, the piezoelectric material facing the interval portion is provided with the non-drive phase electrode so as not to spontaneously cause piezoelectric oscillation, and the common electrode and short circuit lines or the like are short circuited.
The detection phase electrode is an electrode for detecting the oscillation state of the piezoelectric material and may be freely provided insofar as the oscillation is not considerably impaired, and is provided between the two drive phase electrodes in many cases. Distortion arising in the piezoelectric material of the detection phase electrode unit is converted to an electric signal according to the piezoelectric constant of the piezoelectric material, and then output to the detection phase electrode.
A conductive line which inputs/outputs power is disposed in the piezoelectric element and a diaphragm containing an elastic body is further attached thereto. The resultant piezoelectric element is used as a stator. When an alternating voltage is applied to one drive phase electrode of the stator, a standing wave of a wavelength λ arises in the diaphragm over the entire circumference of the diaphragm. When an alternating voltage is applied only to the other drive phase electrode, a standing wave similarly arises. However, since the interval portions are provided as described above, the position thereof is a position which is rotated and moved in the circumferential direction using the length of λ/4 as the unit with respect to the standing wave.
One in which an annular elastic body is brought into contact as a rotor, under pressure, with the surface opposite to the diaphragm of the stator is an annular-shaped oscillatory wave motor.
As another system, an oscillatory wave motor is also mentioned which can be driven by rotating a rotor due to expansion/contraction oscillation of an annular-shaped piezoelectric material in a state where electrodes and a diaphragm are attached to the inner side and the outer side of the piezoelectric material and the rotor is brought into contact with the inner side or the outer side under pressure.
When alternating voltages having the same frequency and having a time-related phase difference of π/2 are simultaneously applied to each drive phase electrode of such an oscillatory wave motor, two generated standing waves are synthesized. As a result, a traveling wave (wavelength λ) of bending oscillation which travels in the circumferential direction arises in the diaphragm.
In this case, each point on the side of the rotor of the diaphragm performs a certain oval movement, and therefore the rotor receives frictional power in the circumferential direction from the diaphragm to rotate relatively to the stator. The rotation direction of the rotor can be reversed by switching the phase difference in the alternating voltage applied to each drive phase electrode to a positive phase or a negative phase.
The rotation speed of the oscillatory wave motor is determined based on the driving frequency (frequency of the alternating voltage to be applied).
When driving the oscillatory wave motor during a stop, the rotation operation is started by applying an alternating voltage having a driving frequency whose frequency is higher than the resonance frequency of the oscillatory wave motor. Then, control of gradually bringing the driving frequency close to the resonance frequency is performed. The rotation speed is further accelerated as the driving frequency is brought closer to the resonance frequency of the oscillatory wave motor. Then, the rotation speed reaches the highest rotation speed at the resonance frequency. Thus, the oscillatory wave motor can perform drive at a desired rotation speed by sweeping frequencies from a frequency region higher than the resonance frequency toward the resonance frequency.
By connecting an appropriate control means to the oscillatory wave motor according to a purpose, a drive control system capable of controlling the rotation speed can be produced. In particular, a phase difference detecting means (also referred to as a phase comparator) which compares phases, and then outputs a voltage value according to the comparison results is connected to the control means in many cases.
In the case where the phase difference detecting means is adopted, when the oscillatory wave motor is driven, an electric signal (1) to be output from the detection phase electrode according to the amplitude of the oscillation arising in the detection phase electrode unit is input into the phase difference detecting means together with an electric signal (2) applied to the drive phase electrode. Then, the deviation degree from the resonance state can be grasped based on phase difference information output from the phase difference detecting means. The frequency of the electric signal to be applied to the drive phase electrode is determined in the control means based on the information, and then a desired traveling wave is generated, whereby the rotation speed of the rotor can be feedback-controlled.
However, the maximum input voltage value of a general phase difference detecting means is variously limited and thus is not large. Therefore, the voltage values of the electric signals (1) and (2) are usually voltage values exceeding the maximum input voltage value of the phase difference detecting means. Therefore, an oscillatory wave motor control system described in Japanese Patent Laid-Open No. 62-85684, for example, is provided with a mechanism (voltage step-down circuit) for stepping down the voltage level to the voltage of the logic level before the electric signals (1) and (2) are input into the phase difference detecting means to step down the voltage.
In the oscillatory wave motor, when the frequency of the alternating voltage to be applied to the drive phase electrode is changed to be lower than the resonance frequency before the feedback control by the control means based on the phase difference information output from the phase difference detecting means is performed, the rotation of the rotor stops (which is also referred to as a cliff step-down phenomenon). In order to prevent the phenomenon, it is devised that a detection permissible error of the phase difference between the electric signals (1) and (2) which can be input into the phase difference detecting means is set to limit excessive lowering of the frequency of the alternating voltage.
In order for the phase difference detecting means to distinguish the voltage values of the electric signals (1) and (2), the voltage values of the electric signals (1) and (2) need to be equal to or lower than the minimum voltage value in terms of the part performance of the phase difference detecting means. It is a matter of course that the phase difference detecting means cannot distinguish voltage values in a range deviating from the range of the resolution and the accuracy of voltage detection values of the phase difference detecting means.
Immediately after the rotation of the oscillatory wave motor starts, i.e., at the early stage of rotation, the drive is started at a driving frequency considerably far from the resonance frequency, and therefore the oscillation amplitude in the detection phase electrode unit is small and the electric signal (1) to be output from the detection phase electrode is small. Accordingly, the lowest rotation speed at which the phase difference detecting means distinguishes the electric signal (1), and the control means can perform the feedback control is determined based on the minimum detection value of the phase difference detecting means.
Hereinafter, the cause of a problem in that the electric signal (1) varies in each oscillatory wave motor is described referring to individual factors.
The traveling wave generated in the oscillatory wave motor does not necessarily have a completely single oscillation mode. Therefore, turbulence of the waveform of the traveling wave arises under the influence of other oscillation modes contained in the traveling wave. Moreover, the influence of the other oscillation modes has individual differences among oscillatory wave motors. Therefore, even when the alternating voltages to be input into the drive phase electrodes are the same, the oscillation amplitude in the detection phase electrode unit varies. Therefore, the electric signal (1) to be output from the detection phase electrode also varies (factor 1).
When a resistance voltage dividing circuit containing at least two resistors is considered as the mechanism of a voltage step-down circuit of stepping down a voltage in order to input the same into the phase difference detecting means, the variation of the factor 1 of the electric signal (1) to be output from the detection phase electrode is further increased due to an individual variation of the resistance values due to a part error of the resistors (factor 2).
Since the electric signal (1) varies due to the factor 1 and the factor 2, the voltage value in the phase difference detecting means of the electric signal (1) which actually allows the control means to perform the feedback control is much higher than the minimum voltage value as the part performance of the phase difference detecting means described above. Therefore, it has been difficult to perform the feedback control from the early stage of rotation in former techniques.
The present invention has been made in view of such a background art and provides an oscillatory wave drive device which can lower the minimum input voltage value at which a control circuit can correctly judge a phase difference, so that feedback control can be performed from the early stage of rotation and an optical apparatus employing the same.